STUDY SHOWS FEMAL FERTILITY DROPS SHARPLY AFTER AGE OF 30

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An unusually large and rigorous study of female fertility appears to demonstrate that the ability of women to become pregnant decreases sharply from the age of 31 to that of 35. The study shows the decline coming earlier and more precipitously than had generally been thought.

The common wisdom had been that the chances for pregnancy were high from the age of 25 to that of 35 and declined rapidly thereafter. But in the new research, conducted by French scientists, the most rapid decline was in the 31- to 35-year-old range, with a lesser decline thereafter in the childbearing years.

An editorial accompanying the report, which apppeared in the Feb. 18 issue of The New England Journal of Medicine, said, ''If the decline in fecundity after 30 is as great as the French investigation indicates, new guidelines for counseling on reproduction may have to be formulated.'' Health and Social Implications

The research has health and social implications for women who, in increasing numbers, are delaying child-rearing to pursue professional careers or for other reasons.

The research was conducted among 2,193 women who were married to sterile men and who were trying to conceive through artificial insemination with donor semen. Over a period of about one year women were inseminated in 12 ovulation cycles and were given pregnancy tests at frequent intervals.

The physicians and scientists who conducted the study assert that this was a more reliable fertility research than had previously been done. The few previous studies had been complicated by such unknown factors as the degree of the husband's fertility, the frequency of the couple's sexual relations, and latent diseases.

The results showed that women 25 and under had a 73 percent rate of successful conceptions, those from 26 to 30 had a 74 percent rate, those from 31 to 35 had a rate of 61 percent, a significant drop of 13 percent from the lower age group. A relatively small group, 144 women, of those older than 35 had a 53 percent rate of success. In the latter group, there were 16 women over the age of 40.

The studies were conducted at 11 artificial insemination centers across France by physicians working with a federation of organizations for the study of human reproduction.

Although the subjects in the study were a special group with special problems, experts in the field said they believed the success in conceiving in this experiment would be roughly similar to that of women studied under different circumstances.

In an editorial in the medical journal, Dr. Alan H. DeCherney and Dr. Gertrud S. Berkowitz of the Yale University School of Medicine cited the importance of the study, noting the changing patterns of childbearing in the United States. They said national statistics showed that, while women 30 or over had contributed 6.8 percent of the country's first live births in 1960, they contributed 8 percent in 1979.

''Although these figures might not suggest a dramatic shift in childbearing patterns, the increase in first as well as second and third births among women in their 30's has been particularly pronounced in recent years, and there is no evidence this trend is shifting,'' they said. Current Counseling Practice

The editorial also notes that current age-and-reproductive counseling is generally limited to the increased risk of Down's syndrome and other genetic abnormalities with advanced maternal age. And the two scientists wondered if, because of the study, women would reverse the current trend and return to having children in their 20's and concentrating on professional careers in their 30's. If so, they said, it might entail restructuring educational and training programs, and the labor market.

Although there is a growing body of knowledge about woman's reproductive functions, the French report, limiting itself to statistical analysis, gave no explanation of possible biological underpinnings for the decline in fecundity they recorded.

The editorial writers pointed out that some loss of fertility can be the result of gynecological diseases such as endometriosis, the failure of the female tubal organs and hormonal disorders. But why from 25 to 47 percent of the women did not conceive is not known.

A version of this article appears in print on , Section A, Page 1 of the National edition with the headline: STUDY SHOWS FEMAL FERTILITY DROPS SHARPLY AFTER AGE OF 30. Order Reprints | Today’s Paper | Subscribe